Centrifugal air classifiers



March 5, 1968 p, EYE ETAL 3,371,782

' CENTRIFUGAL AIR CLASSIFIERS Filed Dec. 8, 1965 2 Sheets-Sheet 1 31 5:x a; .51 K 57 March 5, 1968 Filed Dec. 8, 1965 P. MEYER ETAL CENTRIFUGALAIR CLASSIFIERS 2 Sheets-Sheet '2 United States Patent Office 3,371,782Patented Mar. 5, 1968 3,371,782 CENTRIFUGAL AIR CLASSIFIERS Paul Meyer,19 Paget Road, London, N. 16, England, and Geoffrey Daniel Humphreys,Woodbridge, Ash Road, Hartley, near Dartford, Kent, England Filed Dec.8, 1965, Ser. No. 512,308 Claims priority, application Great Britain,Dec. 11, 1964, 50,647/64; Sept. 8, 1965, 38,374/65 8 Claims. (Cl.209-144) This invention relates to centrifugal air classifiers forground or other solid particles of varying size and/or density.

Rotary air classifiers are known in which a stream of air or othercarrier fluid is induced to flow from the pe riphery of a rotor spirallyinwards to emerge at the eye thereof. Material to be classified iseither introduced into the rotor at some position between the peripheryand the eye or enters the rotor at its periphery together with thecarrier-fluid. In the rotor, the suspension is subjected to an interplayof centrifugal and drag-forces such that, as many as possible of theparticles below a required cut size, the fines, emerge from the eye ofthe rotor while as many as possible of the oversize or rejects areejected at the periphery of the rotor.

To further this action, rotors are fashioned with shrouds such that thedepth of the path of the air increases towards the centre, i.e., withdecreasing radius, centrifugal force decreases to such a degree that,for particles of a given size, the radial drag force component and thecentrifugal force are in the same ratio over the whole flow path betweenperiphery and eye of the rotor. While for the fines, this ratio exceeds1, and for the rejects it is less than one, the ratio is approximatelyunity for the cut size. For this reason there is a tendency for cut sizeparticles to collect in the rotor, particularly if the rotor is mountedon a substantially vertical axis and is given a flat or inward anddownward sloping bottom shroud.

Furthermore, while the above constant ratio type of rotor makes a highdegree of separation possible in principle, this in practice is limitedby the means applied to impose the rotation on the air-stream with itsdust entrained to a greater or smaller degree, as well as by meansemployed particularly in rinsing" the rejects, to eliminate agglomeratedfines therefrom.

Where the particles are introduced as a powder-flow into a constantratio type rotor at some position between the periphery and the eye ofthe rotor (powder fed type), the minimum air quantity, to make efficientseparation possible will be pumped through the rotor to keep down thepower input, both on pumping and on turning the rotor. With the constantratio type rotor, which makes effective separation possible at high dustburdens, this leads to rotors, which are of low depth at theirperiphery, and hence are compact and easy to manufacture.

It is the object of this invention to reduce or to eliminate the abovedrawbacks for constant ratio type air classifiers of the powder fedtype.

It is a further object of this invention to provide a classifier adaptedto be fed directly from air transport systems with very high powder toair ratios, such as air-slide and similar systems.

According to this invention in a rotary air classifier comprising acasing and a rotor mounted to revolve therein and wherein the rotor hasa top plate and a bottom shroud plate and radial blades extendingbetween the two plates, the shroud plate is sloped downwardly andoutwardly to permit gravity to assist in rejection of the particles ator near cut size and not normally rejected by centrifugal forceexceeding the drag exerted by the carrier fluid and means is providedfor introducing material to be classified into the rotor in a directionsuch that it is thrown by centrifugal force across the air streamthrough the classifying zone.

Material to be classified may be introduced to the rotor by means of afeed duct having a central point of discharge whereby the particles aresubjected to the action of centrifugal force before entering theclassifying zone.

Preferably the rotor has a downwardly inclined top plate and the bottomshroud plate is in two sections which are spaced apart to provide anentrant slot for introduction of material to be classified.

The invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a sectional elevation of a constant ratio type powder fedclassifier in accordance with the invention.

FIGURE 2 is a sectional elevation of a classifier having design ofcasing to enable the raw material to be introduced by means of an airslide and provided with a rotor designed to accommodate the increasedair flow.

Referring to the drawings, the classifier comprises a casing indicatedgenerally at 22, mounted to rotate within which is a rotor 2, the rotor2 being fast on a central shaft 3 which is journalled in bearings in ahousing 23 extending upwardly through the conical rejects section of thecasing 22.

The rotor consists of a top plate 52 and a bottom plate, the latterbeingin two sections 534, of which the outer section 4 is curved outwardlyand downwardly, being formed integrally with a horizontal plate 54', bywhich the rotor is secured to the shaft 3. The inner section 53, whoselower perimeter is spaced from the upper perimeter of section 4 to forman annular opening 58 is integral with the lower end of the duct 32. Inplace of the single opening 58 access may be through a series of slots.

The plates 52 and 4-53 which make up the rotor are joined by a series ofradial blades 5. The top plate 52 is curved outwardly and downwardly andit will be seen that the cross section of the rotor blade is such thatthe chamber increases in depth from the perimeter to the centre.Particles of material, when they have entered the rotor are submitted tothe action of centrifugal force operating in opposition to the incomingair stream, the magnitude of which force is directly proportional to thedistance of the particles from the centre of the rotor.

Particles to be classified enter the rotor chamber through a hopper 33whose lower end opens into the axially extending duct 32. Discharge ofthe rejects is through a discharge orifice 15 at the bottom of therejects section 22 while the separated fines and air collect in thecentral space of the rotor chamber and then pass upwardly and out at thetop into a volute chamber 35, having a discharge pipe 36.

Air is admitted through a feed pipe 31 which opens into an annularchamber 34 above the cover plate 21 of casing 22 which is joined byconical section 51 to the underside of volute chamber 35. The inner wallof chamber 34 is constructed in the form of a volute so that the airpressure is evenly balanced. Opening out of the bottom of chamber 34 isa cylindrical passage 57 provided by a flange plate 57 whose lower endterminates immediately opposite the inlet to the rotor. Radial Vanes 52'spaced equidistantly around the top plate 52 reduce leakage of air overthe top of the rotor.

It is a feature of this invention that by extending the feed duct 32 forthe material to be classified to the bottom of the rotor chamber, itenables the articles to be introduced in an area which coincides with acentral section of the classifying zone.

In the embodiment illustrated, where the duct 32 connects with theinverted conical section plate 53'; the plate 53' which is spaced fromthe bottom plate 54 of the rotor, provides the boundary wall of aradially extending flow passage, through which the incoming materialpasses finally entering the interior of the rotor through opening 58 atits perimeter.

By introducing the particles at a central point it secures the advantagethat they are thrown by centrifugal force across the air stream frombelow and at an acute angle with the result that the spreading of thedust is more effective. Cut size particles may rebound from the topplate and under gravity pass down the sloping plate to leave theperiphery of the rotor with the rejects. Such impact also causes some ofthe ultra fines adhering to the rejects to splinter off, which isimportant in rinsing the rejects.

Referring now to FIGURE 2, there is shown an arrangement similar to thatof FIGURE 1 but having a piped connection 60 for a fluidized feedleading directly into the duct 32.

The rotor comprises a bottom plate 54 which is modified in that itextends radially outwards in a horizontal plane for a substantialdistance and beyond the entry slot 58, the perimeter 4 being at an angleinstead of being curved as in FIGURE 1.

It will be seen therefore that inwardly of the entry slot, thecross-sectional area of the rotor has a stepwise or sudden increase indepth, the purpose being to accommodate the additional air flow causedby the fluidised feed, the air flow resulting from which constitutes asubstantial air leak into the rotor.

This stepwise increase is brought about by the inner section 53 beingout of line with the outer section 54.

What is claimed is:

1. A rotary air classifier including a casing provided with an inlet formaterial to be classified and a discharge outlet for classifiedparticles, and a rotor mounted to revolve about a vertical axis withinthe casing, said rotor comprising a top plate and a bottom shroud plate,the classifying zone defined between said plates being subdivided byradial blades, said zone being open at its perimeter for admission ofcarrier fluid from the casing, means for introducing material to beclassified into the rotor in a direction such that it is thrown bycentrifugal force across the air stream in the classifying zone, thefluid and fines discharging through a central orifice in the rotor andthe shroud plate being sloped downwardly and outwardly to permit gravityto assist in rejection of particles near cut size and not rejected bycentrifugal force exceeding the drag exerted by the carrier fluid, meansfor supplying carrier fluid to the casing and means for impartingrotation to the rotor.

2. A rotary air classifier including a casing provided with an inlet formaterial to be classified, a discharge outlet at the bottom forclassified particles and a second discharge outlet for the carrier fluidand fines and a rotor mounted to revolve about a vertical axis withinthe casing, said rotor comprising a top plate and a bottom shroud plate,the classifying zone defined between said plates being subdivided byradial blades, said zone being open at its perimeter to admission ofcarrier fluid from the casing, means connecting said inlet to the rotorfor feeding material into the classifying zone and so arranged that thematerial on entering the rotor is thrown by centrifugal force across thecarrier fluid stream passing through the classifying zone, said shroudplate being sloped down- 4 wardly and outwardly to permit gravity toassist in the rejection of particles near cut size and not rejected bycentrifugal force exceeding the drag exerted by the carrier fluid, meansfor supplying carrier fluid to the casing and means for impartingrotation to the rotor.

3. A rotary air classifier including a cylindrical casing having aconical rejects section in which is a discharge outlet for classifiedparticles, said casing being provided with an inlet for introduction ofmaterial to be classified and an inlet for carrier fluid, saidclassifier comprising a drive shaft mounted to revolve about a verticalaxis concentric with that of the casing, a rotor fast on said shaft andincluding a top plate and a bottom shroud plate, the classifying zonedefined between said plates being subdivided by radial blades, saidclassifying zone being open at its perimeter for inflow of carrier fluidfrom the casing, means including a duct connecting the feed inlet formaterial to the rotor and so arranged that on entering the retor thematerial is thrown by centrifugal force across the stream of carrierfluid passing through the classifying zone, said bottom shroud platesloping downwardly and outwardly to permit gravity to assist inrejection of particles near cut size and not rejected by centrifugalforce exceeding the drag exerted by the carrier fluid, means connectingsaid inlet to a source of carrier fluid and means for imparting drive tosaid shaft.

4. A rotary air classifier as claimed in claim 3 comprising a rotorwherein the bottom shroud plate includes inner and outer sections spacedapart to provide an entrant slot to the classifying zone and wherein theinlet duct for material unites at its lower end with the inner sectionso that incoming material enters the rotor beneath the inner section ofthe shroud plate.

5. A rotary air classifier as claimed in claim 4 when used for treatmentof the material in a fluidized condition characterised in that the innerand outer sections of the shroud are axially spaced so that the flowsection of the rotor inwardly of the entrant slot for raw material has astepwise increase to accommodate the additional air flow from thefluidized bed.

6. A rotary air classifier as claimed in claim 3 comprising a rotorhaving a downwardly inclined top plate.

7. A rotary air classifier as claimed in claim 3 in which the inlet formaterial to be classified comprises a feed hopper on the casing, anaxial duct open at one end to receive material from the hopper and atits other end discharge into the rotor and in which the bottom shroudplate is in two sections, the inner section being attached to the saidduct and the outer section being fast on the motor shaft and spaced fromthe inner section to provide an entry slot for material to beclassified.

8. A rotary air classifier as claimed in claim 7 in which the outersection of the shroud plate comprises a plate that lies radially to therotor shaft and combines with the inner section to provide a passageleading from said duct to the entry slot of the rotor.

References Cited UNITED STATES PATENTS 2,199,015 4/1940 Toenfeldt 209144X 2,276,761 3/1942 Carey 209144 2,338,779 1/1944 Mutch 209-144 FRANK W.LUTTER, Primary Examiner.

TIM R. MILES, Examiner.

1. A ROTARY AIR CLASSIFIER INCLUDING A CASING PROVIDED WITH AN INLET FORMATERIAL TO BE CLASSIFIED AND A DISCHARGE OUTLET FOR CLASSIFIEDPARTICLES, AND A ROTOR MOUNTED TO REVOLVE ABOUT A VERTICAL AXIS WITHINTHE CASING, SAID ROTOR COMPRISING A TOP PLATE AND A BOTTOM SHROUD PLATE,THE CLASSIFYING ZONE DEFINED BETWEEN SAID PLATES BEING SUBDIVIDED BYRADIAL BLADES, SAID ZONE BEING OPEN AT ITS PERIMETER FOR ADMISSION OFCARRIER FLUID FROM THE CASING, MEANS FOR INTRODUCING MATERIAL TO BECLASSIFIED INTO THE ROTOR IN A DIRECTION SUCH THAT IT IS THROWN BYCENTRIFUGAL FORCE ACROSS THE AIR STREAM IN THE CLASSIFYING ZONE, THEFLUID AND FINES DISCHARGING THROUGH A CENTRAL ORIFICE IN THE ROTOR ANDTHE SHROUD PLATE BEING SLOPED DOWNWARDLY AND OUTWARDLY TO PERMIT GRAVITYTO ASSIST IN REJECTION OF PARTICLES NEAR "CUT SIZE" AND NOT REJECTED BYCENTRIFUGAL FORCE EXCEEDING THE DRAG EXERTED BY THE CARRIER FLUID, MEANSFOR SUPPLYING CARRIER FLUID TO THE CASING AND MEANS FOR IMPARTINGROTATION TO THE ROTOR.